1. Field of the Invention
The present invention relates to, in an outboard motor, an engine case that houses an engine and so on constituting a power unit.
2. Description of the Related Art
Major types of a propulsion unit or a propulsion system for a craft or boat include an outboard motor, an inboard-outdrive motor, an inboard motor, and the like. The outboard motor, also called an outboard drive or the like, is integrally made up of an engine, auxiliary machines, gears and shafts of a drive system, a screw, and so on, and is mounted onto a transom board of a stern of a hull in general. Typically mounted in a small boat or the like, the outboard motor has a steering function and a tilting function.
Further, the inboard-outdrive motor, also called an inboard engine-outboard drive or the like as an installation method for a propulsion unit of a small craft or the like, has an engine mounted at an inboard stern portion and a drive unit made up integrally of reduction gears, a forward and reverse clutch, a propeller, and so on disposed on an exterior of the transom board.
Furthermore, the inboard motor is one of installation methods typically for a propulsion unit of a small craft or the like. Also called an inboard drive, it is a method placing an engine, reduction gears, and a forward and reverse clutch at an inboard central portion or the like, extending a propeller shaft toward the stern, and placing the propeller under the water from a craft bottom portion. A rudder determining a traveling direction of the craft is often placed behind the propeller.
As above, the outboard motor, the inboard-outdrive motor, and the inboard motor differ in engine mounting structure in the boat, so that each has an advantage and a disadvantage. In terms of user-friendliness or the like, for example, as for the inboard motor or inboard-outdrive motor, its engine chamber occupies part of a hull, so that an accommodation space is reduced to make its user-friendliness deteriorate. On the other hand, the outboard motor is placed outside the accommodation space, so that its user-friendliness in the hull is good, but regarding boarding or getting off via a stern and carriage of a thing (a fish, a net, a fishing line, or the like) via a stern, the user-friendliness deteriorates.
Further, in terms of watertight sealing that is quite important in this type of boat, an engine of the inboard motor or inboard-outdrive motor is mounted by being hung from a hole of a ceiling of the engine chamber. A ceiling cover of the engine chamber is just put on the engine chamber, and does not include a sealing member or the like in particular, so that water easily enters the engine chamber from a ceiling surface. Further, in the inboard-outdrive motor, the engine is placed in the hull, and a propulsion unit is placed outside the boat, so that a part coupling the engine and the propulsion unit is required. Thus, a large hole is opened in a transom board, and the engine and the propulsion unit are coupled. Watertightness of the above part relates to rigidity or flatness of the transom board, which affects securement of watertightness remarkably. The above watertightness is quite important not only for the inboard motor and inboard-outdrive motor, but also for the outboard motor.    [Patent Document 1] Japanese Patent No. 3038606    [Patent Document 2] Japanese Patent No. 2652861    [Patent Document 3] Japanese Laid-open Patent Publication No. 2001-173443    [Patent Document 4] Japanese Patent No. 2984027    [Patent Document 5] Japanese Patent No. 3054962    [Patent Document 6] Japanese Laid-open Patent Publication No. 2003-120378
Particularly, with respect to the above watertightness, more concretely, in the structure in Patent Document 1, an engine (2) in a hull and a propulsion unit (8) outside a boat are coupled via a universal joint (7). A large gap or space is required around the universal joint (7), and corresponding to the gap or space, a large hole is opened in a transom board (5) at a stern of the hull, and it is not easy to secure watertightness of a portion of the above hole.
Further, in Patent Document 2, an engine (12) is rubber-mounted in a hull (11). Exhaust is required to be guided to the outside of a boat from the engine, and the engine fluctuates because of it being rubber-mounted, so that flexible parts (23, 24) made of rubber or the like are required in an exhaust system (19, 21, 22). Also in the exhaust system, there is required a hole (19) passing through to the outside of the boat from the inside of the boat, and the above hole is positioned lower than a drive shaft in the case of exhaust being discharged into the water, resulting in that it is not easy to secure watertightness.
Further, particularly, a muffler constituting an exhaust device affects exhaust performance, sound deadening performance, and further engine performance, and so on, and how the muffler is disposed and structured is quite important.
In Patent Document 3, for example, it is structured that exhaust of an engine of an inboard-outdrive motor is guided to the outside of a watercraft through a catalyst (64) and a muffler (66). Unlike a vehicle, exhaust of a marine engine stays in a hull, and thus is not directly exposed to traveling air or the like, so that the exhaust is not cooled down easily. In this example, by a pump (42), air is pressurized to be supplied between an exhaust conduit (26) and a protective conduit (32) positioned around the exhaust conduit, and thereby the exhaust is cooled down.
Further, in Patent Document 4, similarly to a conventional outboard motor or inboard-outdrive motor, exhaust is discharged into the water, or is directly discharged into the air. In both the cases, a wet method to inject seawater into the exhaust is employed. In the above wet method, a surrounding heat insulation pipe for reducing the temperature of an exhaust system is not required, so that the exhaust system itself is not increased in size, and further it is also possible to reduce sound speed or sound pressure.
However, in Patent Document 3, even though the exhaust is cooled down as described above, the exhaust conduit (26) is needed to be lengthened in order to sufficiently cool down the muffler (66), and the sufficient volume of an installation part is required. The protective conduit (32) has to be increased in diameter with respect to the exhaust conduit (26) in order to obtain an air heat insulation layer, so that the volume occupied by an exhaust system in the hull is increased and it is difficult to secure an effective space in the watercraft.
Further, in Patent Document 4, in a method of underwater exhaust discharging, an engine (12) is fixed to a hull (11), and a propulsion unit (13) is moved when tilting, steering, or the like, so that a flexible part (bellows) is needed for an intermediate portion of the exhaust system.
Further, in a bellows structure in Patent Document 5, a bellows part (46) deteriorates by an exhaust component and is bent by steering and tilting, so that it is difficult to maintain its watertightness for a long period of time and the bellows part (46) is required to be replaced regularly. Further, when the bellows part (46) being replaced, a propulsion unit portion (7) has to be removed, so that man-hour is increased. On the other hand, in a method of aerial exhaust discharging, sound speed and sound pressure are reduced, but it is not possible to obtain sufficient sound deadening performance because a muffler is not provided.
Further, in Patent Document 6, in a typical exhaust structure of an outboard motor, an engine (7) and an exhaust outlet port (80) are integrally moved when steering or tilting, so that exhaust can be discharged into the water without a bellows structure. However, an exhaust system (46, 80) is placed integrally with the engine (7), so that it is difficult to place a catalyst having a sufficient size. Further, a lower end of the engine (7) (a bottom portion cylinder) and the water surface position are close to each other when a ship stops, so that deterioration of a catalyst and an oxygen sensor caused when being covered with water is a problem.